The present invention generally relates to cooling systems. More particularly, the present invention relates to apparatus, systems and methods for providing cooling system load control using a random start of a first defrost cycle.
For certain applications, a centralized refrigeration system may be suitable to service multiple locations requiring temperature regulation. One such application is galley cooling on a large commercial aircraft. In this application, a vapor cycle system comprising of a compressor, condenser, expansion valve, evaporator, and refrigerant is often utilized to chill liquid coolant, which is cooled through a centralized evaporator for distribution to various cooling units in the galleys of the aircraft. This coolant may flow through heat exchangers in the cooling units. By using a fan or air convection, the heat exchanger may absorb heat inside the cooling units, thereby reducing the inside ambient temperature to a desired food safe temperature. The coolant then returns to the vapor cycle system evaporator to be cooled again.
To keep food stored in the galleys at a safe temperature, all of the cooling units may need to be started or powered on at the same time. Further, due to the configuration of the electrical wiring in the aircraft, simultaneous power on of the cooling units may be an operational requirement. However, if there is no coordination between the cooling units, then all of the cooling units may enter a periodic defrost cycle at approximately the same time. During the defrost cycle, the cooling units may engage a valve to stop the flow of coolant through the heat exchangers. When all of the cooling units simultaneously stop the flow of coolant, the lack of coolant flow and heat load causes a rapid drop in the evaporating pressure of the vapor cycle system, which in turn causes a rapid increase in compressor pressure ratio and compressor outlet temperature.
To prevent overheating of the compressor, the vapor cycle system may require shutdown during the defrost cycle and startup after the defrost cycle ends. This repeated startup and shutdown cycle causes thermal stress and mechanical stress (wear and tear) on system equipment that can cause premature failure. Moreover, orchestrating this startup and shutdown cycle is a delicate and challenging process when conducted in the extremely hot or cold environment of an aircraft on ground or in flight.
Some form of cooling system load control is therefore needed to desynchronize the defrost cycles, thereby maintaining a baseline coolant flow and avoiding wear and tear from unnecessary start and stop cycles. One prior approach staggers the startup of the cooling units to prevent defrost cycles from starting at the same time. However, this approach is undesirable in the context of food storage, as food may be kept at unsafe temperatures when cooling units are not powered on right away.
Furthermore, the prior approach may require modifying the software logic and wiring in the refrigeration system. This software modification may not be feasible, particularly when using an existing turnkey solution. The additional system software complexity may also make integration, maintenance and repair more difficult, which is particularly troublesome in a commercial aircraft where every minute of downtime counts.
As can be seen, there is a need for apparatus, systems and methods for cooling system load control that are robust in challenging operational environments while still providing ease of integration and simplified management.